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Herpetofauna in Riparian Habitats
Along the Colorado River in Grand Canyon 1
2
Peter L. Warren and Cecil R. Schwalbe
Abstract.--Lizard
population
densities
and
species
composition
were sampled in riparian and non-riparian
habitats along the Colorado River. The highest densities
were found in shoreline habitats, moderate densities in
riparian habitats and lowest densities in non-riparian
habitats.
Rapidly fluctuating river flow levels may have a
deleterious
effect on lizard populations by
trapping
populations on alluvial bars and inundating nest sites.
For
years
riparian habitats have
been
recognized
as making a contribution to
the
structural diversity and species richness
of
natural communities that exceeds the relative
areal extent of those habitats.
The availability
of
additional water permits growth of plant
species and growth forms that are lacking in the
surrounding upland vegetation.
Their occurrence
in
turn provides food and habitat resources
without which some animal populations may not
otherwise persist in the upland community.
To
most biologists these patterns are obvious, but in
many
cases
they
are
surprisingly
poorly
documented.
densities
higher.
upland vegetation may
actually
be
One group that has received relatively little
attention with respect to the importance
of
riparian habitats to their density and diversity
is the reptiles. It is common to find comments in
the literature about the higher density of some
species in riparian sites (Lowe and Johnson, 1977;
Vitt and Ohmart, 1977; Tinkle, 1982) and some
studies of lizard demography have been performed
in riparian areas (Tinkle,
1976; Tinkle and
Dunham, 1983; Vitt and Van Lohen Sels, 1976).
However, quantitative studies comparing reptile
density and diversity in riparian and adjacent
non-riparian habitats are few.
Only recently has
emphasis on riparian ecosystems has begun to
address effects of management practices and exotic
riparian
vegetation
on
riparian
reptile
communities (Szaro et al., 1985; Jakle and Getz,
1985; Jones and Glinski, 1985).
Some of the best studied examples of the
contribution of riparian habitat to local species
density and diversity are for birds and mammals.
Gallery forests of cottonwood and willow along
some Southwestern rivers have been shown to have
some of the highest densities of nesting birds in
North America, much higher than in surrounding
semiarid upland sites (Johnson et al., 1977;
Anderson, Higgins and Ohmart, 1977).
Riparian
habitats contribute breeding sites, feeding areas
and migratory routes for birds.
Hammal species
diversity is also higher along watercourses, where
some species find necessary cover that is lacking
in more open adjacent arid vegetation (Anderson,
Drake and Ohmart, 1977),
although small mammal
1
Paper presented at the First North
Riparian
Conference
[University of
Tucson, April 16-18, 1985].
in
The present study was designed to examine the
patterns
of distribution of reptile
species
relative to riparian habitats along the Colorado
River in Grand Canyon National Park. This work is
part of a larger study to determine the effects of
fluctuating flows from Glen Canyon Dam on plant
and animal populations in and along the Colorado
River.
Data presented here were gathered during
constant flow levels of approximately 40,000 cubic
feet per second (cfs) in June and 25,000 cfs in
August, 1984.
Additional censuses will he made
during lower,
fluctuating flow levels.
The
results presented here are from the first year of
a multi-year project, and are restricted to only
those
species for which the most data were
gathered, the diurnal lizards.
American
Arizona,
2
Peter L. Warren is Research Assistant at the
Arizona Remote Sensing Center, Office of Arid
Lands Studies, University of Arizona, Tucson,
Arizona.
Cecil R. Schwalbe is Herpetologist with
Nongame Branch, Arizona Game and Fish Department,
2222 W. Greenway Road, Phoenix, Arizona.
347
STUDY AREA
Habitats Sampled
We censused lizard populations at a series of
sites along the Colorado River above and in Grand
Canyon National Park beginning near Lees Ferry and
extending downstream 220 miles almost to Diamond
Creek.
The
elevation
at river
level
is
approximately 945 meters (3,100 feet) at Lees
Ferry and drops to 427 meters (1,400 feet) at the
last census locality at mile 220.
The vegetation
through which the river flows is generally Mohave
desertscrub.
However,
there
is
a
gradual
transition in species composition from more coldtolerant species at the upper end of the study
area to a flora composed of many frost-sensitive
species at the lower end (Warren et al., 1982).
Sampling
was performed in ten different
habitats that are distributed in three zones
relative to the river.
The first zone comprised
shoreline habitats within 5 meters of the river
shore. The second zone included all riverine
riparian vegetation greater than five meters from
the river shore.
The third zone included nonriver habitats, both upland and riparian (Table
1).
Three distinct habitats were sampled in the
river shoreline zone.
These were cobble shore,
rocky shore, and cliff faces at the water's edge.
In all cases shoreline samples were characterized
by low vegetation cover, usually less than ten
percent.
Cobble shores generally were characterized by num~rous rocks less than 0.5 meters in
diameter and rounded by erosion. Larger, uneroded
boulders were absent and large patches of bare
sand were occasional. Cobble shores generally were
found at the mou~hs of tributary canyons where the
coarse alluvium that forms level cobble bars was
washed into the river.
The riparian corridor along the river is
characterized by two vegetation zones that are
more or less distinct in species composition and
distribution.
Previous to the construction of
Glen Canyon Dam in 1963 the river channel was
scoured by floods on a regular basis, and the only
riparian vegetation occurred as a belt along the
high water line where flood disturbance was at a
m1n1mum.
Since dam construction lack of largevolume flooding has permitted plants, many of them
exotics, to grow along the water's edge (Turner
and Karpiscak, 1980).
The resulting pattern is
one in which the original riparian vegetation,
consisting
largely
of
mesquite
(Prosopis
glandulosa) and cat-claw acacia (Acacia greggii),
is perched on talus slopes and alluvial terraces
several meters above the current normal water
level.
The new riparian vegetation, dominated by
tamarisk
(Tamarix
chinensis)
and
arrowweed
(Tessaria sericea), occupies sand and cobble bars
along the water's edge.
Table 1.--Location of study sites at which lizard
transect sampling was performed. The number
of habitats sampled in each vegetation zone
is indicated for each site.
Site
Name
Lee's Ferry
Badger
none
none
North Canyon
none
Saddle Canyon
Nankoweep
Kwagunt
Cardenas
Cremation
none
Crystal
Bass
Elves Chasm
Forster
Tapeats
none
Kanab
National
Stairway
none
Whitmore
Parashant
Granite Park
Three Springs
220 mi. Canyon
METHODS
Visual belt transects, modified from the
Emlen (1971) bird census technique, were used to
census the
common diurnal species (Lowe and
Johnson, 1977). This method involves walking a
transect
through representative areas of the
target habitats and recording all individuals
observed within a belt of predetermined width of
four meters.
Transect length varies with size of
the habitat patch, but usually varied from 100 to
300 meters in length.
Transect sites
were
selected to sample a range of variation within
old- and new-riparian habitats and in adjacent
non-riparian desertscrub.
The time of day at the
beginning and end of each transect was recorded as
well as a temperature profile consisting of soil
surface temperature, air temperature at 5 mm and
air temperature at 1.5 m. Weather conditions such
as cloudiness and wind speed were also noted.
As each individual lizard was sighted, the
distance along the transect and the substrate upon
which it was first observed were recorded, as well
as its sex and age, when possible. The substrate
categories used were bare soil, litter, rock (less
than one meter diameter), boulder (greater than
one meter diameter), cliff face, or tree.
When
individuals were in a tree, the tree species and
height above ground were also recorded.
348
River
Mile
-1R
8R
16L
20R
20.5R
43.5L
47R
53R
56R
71L
86L
94L
98R
108.5R
116.5L
123L
134R
140L
143.5R
166L
171R
185R
188R
198R
209L
216L
220R
Shoreline
1
1
1
1
1
1
2
1
1
River
Riparian
NonRiver
1
1
3
3
2
1
4
1
1
1
1
1
2
1
1
2
1
1
1
2
1
1
1
1
1
1
1
1
4
1
2
3
1
3
3
1
1
Total Transects
24
36
8
Total Transect
Length (meters)
2665
5522
2420
can be considered "new-zone" or post-dam habitats.
These were open tamarisk with 15 to 40 percent
cover, dense tamarisk with 60 to 100 percent
cover, and arrowweed with cover similar to the
open tamarisk.
In contrast, rocky shores were composed of
rock fragments of varying sizes ranging from
cobbles up to boulders several meters in diameter.
These shores were generally composed of uneroded
talus and rockfall debris and may include pockets
of bare sand of varying sizes that were trapped
among the boulders.
In contrast to the level
cobble shores, rocky shores usually fell steeply
to the water's edge and were commonly very rugged
and irregular.
Finally, two habitats were sampled in the
non-river zone.
These were desertscrub on canyon
slopes generally ranging from 15 to 30 percent
slope with 15 to 30 percent vegetation cover, and
non-river
riparian
habitats along
perennial
tributary streams.
Sandy shores and heavily vegetated shores
were examined but not sampled systematically for
several reasons.
Heavy vegetation immediately at
the water's edge was relatively uncommon. In most
locations where dense cover was present near the
shore it occured on sandy soil.
Frequently
erosion of sandy soil along the river's edge kept
the immediate shoreline free of dense cover even
though adjacent sandy bars were thickly vegetated.
Open sandy shorelines that lacked vegetation or
rock cover were found to be almost completely free
of reptile and amphibian activity, and although
such sandy shores were spot-checked repeatedly, no
systematic transects were sampled.
RESULTS AND DISCUSSION
Sampling was performed during 18 days in June
and five days in August, 1984.
A total of 68
transects were sampled at 27 localities, with
between one and five habitats sampled per locality
(Table 1).
Preliminary habitat assessments were
made during September, 1983 and April, 1984.
Five common diurnal lizard species
were
successfully
sampled using the belt transect
method. One lizard species (Holbrookia maculata),
two
toad
species (Bufo
punctatus
and
B.
woodhousei), one frog species (Hyla arenicolor~
and
three snake species
(Crotalus
viridis,
Masticophis taeniatus,
and ~
flagellum)were
encountered in numbers too small for adequate
conclusions to be drawn concerning distribution
patterns.
Within
the riverine riparian zone
five
habitats were sampled.
These included two that
can be considered "old-zone" or pre-dam habitats,
mesquite/acacia
alluvial
terraces
and
mesquite/acacia talus slopes. The remaining three
Table 2.--Distribution
of
lizard
occurrence
on
different
substrates along the Colorado River in Grand Canyon, June and
August, 1984.
Numbers in parentheses indicate the percent
of individuals of each species that were observed on each
substrate.
Species
Litter
Uta
stansburiana
2
71
2
( 1. 3)
78
(82.1)
3
(3.2)
9
(9.5)
Sceloporus
magister
11
(12.5)
3
(4.9)
Crotaphytus
insularis
0
Sauromalus
obesus
0
Holbrookia
maculata
0
Total
70
Substrate
Rock Boulder
<lm
>1m
( 1. 3) (46.7) (47.3)
Cnemido,ehorus
tigris
Urosaurus
ornatus
Bare
Soil
4
(4.2)
7
34
11
(12.5) (8.0) (38.6)
Cliff
Tree
1
(0.7)
4
(2.7)
0
150
95
( 1.1)
22
3
(3.4) (25.0)
1
16
27
9
( 1.6) (14.7) (26.2) (44.3)
4
2
1
(14.3) (57.1) (28.6)
1
Total
5
(8.2)
88
61
0
0
7
0
0
1
(100)
0
0
1
1
0
0
0
0
1
95
58
40
32
403
(100)
25
162
349
Substrate Preference
Patterns of Density and Diversity
The lizards showed strong species-specific
patterns of substrate preference (Table 2).
All
species
were observed most frequently on
a
substrate
different from any other
species,
although four species were commonly observed along
a single transect in one habitat.
The most striking observation was the large
differences in lizard densities among the habitats sampled (Table 3).
Total lizard densities
were highest in shoreline and open, "new zone"
riparian habitats and lowest in desertscrub, with
intermediate densities in "old zone" sites.
Most
of the individual species followed the same pattern with highest densities in shoreline and "new
zone" habitats and lowest density in desertscrub.
The only exception to this pattern were collared
lizards which, although relatively rare, were seen
more
commonly in desertscrub than any other
habitat.
Side-blotched lizards (Uta stansburiana) were
the most common species observed as well as the
smallest.
Utas were found predominately in open
sites and the substrates upon which they were most
frequently observed were rocks less than one meter
in diameter and bare soil. They were almost never
seen at a distance greater than one meter away
from cover in the form of rocks or small shrubs.
The
pattern
of differences
in
lizard
densities among habitats was stable through time
as shown by comparison of June and August data
(Fig. 1).
Regression analysis of density data
gathered in the same habitats during two months
indicates that although overall observed densities
declined from June to August, the ranking of habitats based on density remained the same.
This
was possibly the result of cooler,
cloudier
weather encountered during the August census and
consequent lower activity levels of some species.
Whiptails and desert spiny lizards both declined
in observed densities by approximately one-half
between the two census periods.
Western
whiptail
lizards
(Cnemidophorus
tigris), the second most abundant species, were
found most frequently on bare soil or litter.
They frequently occurred in the same habitats with
Uta, but were rarely seen perched on small rocks
ag- Uta does. Cnemidophorus was the only species
observed commonly roaming up to several meters
across bare sand away from cover.
Desert spiny lizards (Sceloporus magister)
were
approximately
equal
in
abundance
to
Cnemidophorus, although they were less noticeable
due to more sedentary habits and preference for
cryptic
substrates
with a
strong
vertical
component, such as large boulders and trees.
Desert spiny lizards were seen most commonly on
boulders larger than one meter in diameter, and
usually on those with fractures and crevices.
At
sites that lacked boulders but had trees, such as
tamarisk stands on sand bars, this species was
also found on larger tree trunks.
On those
occasions when they were observed on the ground,
they were almost invariably at the immediate base
of a large tree or boulder.
Comparison of density values derived from
visual transects in this study with density data
available in the literature is difficult for
several reasons. First and most important is that
our visual census does not attempt to account for
every lizard in the study site as a mark/recapture
study on a permanent grid does. Visual transect
estimates will therefore generally be lower than a
comparable
mark/recapture
estimate.
Second,
lizard densities vary to large degree between
sites, between years, and even seasons or days, at
a single site. Thus any comparison of densities,
regardless of the sample technique, is fraught
with problems unless the sampling is performed
simultaneously at all sites to be compared.
With
these problems in mind, it is still useful to
compare our results with those density data that
are available in the literature.
Tree lizards, Urosaurus ornatus, were also
found on substrates with a strong vertical component. However, they showed a clear preference for
sheer, vertical rock faces on cliffs or large
boulders.
The highest densities of tree lizards
were found on cliff faces that dropped vertically
into the river, usually along eddies or quiet
stretches.
They often sat less than one meter
above water level, just above the splash zone, on
faces that had no fractures or other protection
and that were up to 20 to 40 meters away from the
nearest water-level alluvial soil.
In general the lizard densities observed
along the Colorado River fell within the range of
values that have been observed for these species
in other areas (Table 4).
That species which we
observed
to occur in the
highest
density,
Urosaurus ornatus, was also reported by several
authors to have the highest density of lizard
species studied.
Similarly, of the four most
common species, we generally found Sceloporus
magister to have the lowest density. This species
was reported by several authors usually to have
lower densities than the other three
common
species.
These results indicate that visual
transect
data
are roughly
comparable
with
mark/recapture data.
Black
collared
lizards,
Crotaphytus
insularis, and chuckwallas, Sauromalus ohesus,
were observed much less frequently than the four
preceding
species.
These two species
also
differed from the others insofar as both species
were seen more often in desertscruh than in the
riparian corridor.
Collared lizards generally
were observed perched on rocks or small boulders
that were approximately one meter in diameter or
slightly smaller. Chuckwallas rarely were seen on
transects, hut additional observations indicated
that they prefered deeply fractured boulders and
rock outcrops.
The observed average June densities of 858
lizards per hectare on shoreline cliff-faces
and
300 lizards per hectare in non-river riparian
habitats equal or exceed lizard densities reported
350
Table 3.--Lizard densities in habitats along the Colorado River in Grand
Canyon, Arizona during June and August, 1984.
Values indicate
number of individuals encountered per hectare.
Habitat
Lizard S2ecies
CnemiScelo2- Urosaurus
do,ehorus ~
All
Lizards
Month
Uta
Rocky Shore
June
Aug.
48
20
23
0
60
0
20
100
0
0
150
120
Cobble Bar
June
Aug.
68
60
40
18
15
13
0
0
3
0
125
90
Cliff Face
June
Aug.
0
0
0
0
0
0
858
223
0
0
858
223
Shoreline
Crota2hytus
(<5m2:
River RiEarian (>Sm~:
New Zone
Open Tamarisk
June
Aug.
53
53
78
60
55
60
13
0
0
0
195
173
Arrowweed
June
Aug.
35
33
35
18
5
18
0
0
0
0
73
68
Dense
Tamarisk
June
Aug.
0
13
no sample
40
0
0
53
Terrace
June
Aug.
30
0
15
0
15
13
3
25
1
0
65
38
Talus
June
Aug.
28
10
no sample
15
0
0
53
Desertscrub
June
Aug.
18
5
8
5
5
0
0
0
2
5
30
15
Riparian
June
Aug.
25
208
0
0
125
0
150
0
0
0
300
208.
June
Aug.
35
30
25
13
23
13
10
23
0.7
1
Old Zone
-----
Non-River:
Grand Mean
(All habitats)
in
the literature for
any
habitat.
This
observation is of particular interest considering
the expected under-estimate of visual
census
compared
to mark/recapture methods
discussed
above.
The lizard
densities we observed in
riparian habitats along the Colorado River were
higher
than those in most habitats
thusfar
documented in the Southwest.
They were up to an
order
of magnitude higher than densities in
desertscrub immediately adjacent to the river
corridor.
The most likely explanation for these
densities
is an increased abundance of
93
80
resources.
Many shoreline sites appear to have
much greater numbers of insects than non-riparian
areas for two major reasons. First, debris washed
up along the water's edge in eddies and backwaters
is frequented by many insects.
Second, many
riparian plant species support a larger insect
fauna than non-riparian species (Stevens, 1976).
The
highest local lizard densities
observed
anywhere along the river were both at sites along
the shoreline where lizards were feeding upon
insects.
The highest density was observed at
Cardenas where a total of eight Cnemido,ehorus
tigris and five Scelo2orus magister were observed
feeding
along the shoreline in an area
of
high
food
351
Table 4.--Comparison of average lizard densities in Grand Canyon with those from
other localities.
Ranges are shown in parentheses.
In some cases the
range of values are from replicate sampling in adjacent sites, and in some
cases from sampling in different years. The range of values is not
published in all cases.
Species
Jta
stansburiana
Cnemidoehorus
tigris
Sceloeorus
magister
Urosaurus
ornatus
Total
Lizards
Average Density
(Number/Ha)
Location
Source
Texas
Ariz. desertscrub
Ariz. mesquite
Ariz. riparian
All habitats
Tinkle, 1967
Vitt & Van Loben Sels 1976
12 (8-18)
8 (3-15)
17
30
114 (45-184)
3
12
32
32
7
19 (0-78)
Nevada
Texas
Colorado
Nevada
Texas
Ariz. grassland
Ariz. desertscrub
Ariz. mesquite
Ariz. riparian
Ariz. dry wash
All habitats
Turner, et al, 1969
Degenhardt, 1966
McCoy, 1965
Tanner & Jorgensen, 1963
Milstead, 1965
Lowe & Johnson, 1977
Vitt & Van Loben Sels, 1976
15
10
25
25
18 (0-125)
Utah, riparian
Ariz. desertscrub
Ariz. mesquite
Ariz. riparian
All habitats
Tinkle, 1976
Vitt & Van Loben Sels, 1976
158 (131-188)
101 (42-161)
370
185
16 (0-858)
Ariz., spring
Ariz., summer
Ariz. mesquite
Ariz. riparian
All habitats
Tinkle & Dunham, 1983
6 (2-12)
55
66
8
593
277
89
12
86 (15-858)
Southwest deserts
Pianka, 1967
Ariz. riparian
Lowe & Johnson, 1977
II
Ariz. grassland
"
II
Ariz: Chihuahuan desert II
Vitt & Van Loben Sels, 1976
Ariz. mesquite
II
II
Ariz. riparian
II
II
Ariz. Sonoran desert
II
Ariz. dry wash
"
All habitats
This study
140 (62-238)
22
7
7
33 (0-208)
II
II
II
II
This study
II
II
II
II
II
II
....
This study
II
II
II
"
This study
II
II
Vitt & Van Lob en Sels, 1976
II
II
This study
'·/
approximately three by seven meters, or a density
equivalent to 6,500 lizards per hectare!!
In
spite of their close proximity to one another, no
antagonistic interactions were observed between
individuals of either species, all of which were
active in the area for an hour.
The second
highest density was observed on a vertical rock
face at the waterline on which eight Urosaurus
ornatus were observed in an area of two by twentyfive meters, or 1,600 per hectare.
Again, they
were feeding on insects at the waters edge.
habitats that contained a mosaic of bare sand and
cover such as cobbles and small shrubs. Uta
juveniles were the most common and were often seen
on cobble bars and shoreline.
Tinkle's (1967)
observations that average first-year dispersal of
juvenile Utas is less than six meters suggests
that these--habitats are the location of higher
reproductive activity than non-riparian sites.
Future study of nest site selection will clarify
the
level of reproductive activity
in
the
different habitats.
Reproductive activity of lizards along the
Colorado River was not evaluated directly, but
indirect evidence of reproduction was inferred
from the distribution of first year immature
individuals.
The greatest number of immature
lizards were observed in shoreline and riparian
The distributions of several of the lizard
species studied were consistent with the concept
of "preferential" riparian species as used by
Johnson et al. (1984) in their discussion of plant
species distributions.
Urosaurus, Cnemidoehorus,
Sceloeorus
and
Uta
could
be
considered
352
/
•
second, rising water during the breeding season
from May to July may inundate nest sites in shoreline and riparian-zone sand.
/
/
ACKNOWLEDGEMENTS
/
This research was supported by the Bureau of
Reclamation
and
the Arizona Game and
Fish
Department as part of the Glen Canyon Environmental Studies through cooperative agreement 4-A6-4001810. The National Park Service provided collection permits. Park Service personnel, especially
John Thomas, Steven Carothers, James Gaddy, and
Jon Dick, assisted with logistics.
Many Arizona
Game and Fish personnel contributed to various
aspects of project initiation and field work
including James C. DeVos, Jr., Terry Johnson, Ray
Lee, Henry Maddux, William Persons, and Bruce
Taubert.
Terry B. Johnson reviewed the manuscript.
Humphrey
Summit Associates provided
cheerful, professional logistic support.
/
>.
/.
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c:
(1)
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(1)
c:
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J
LITERATURE CITED
Anderson, B.W., A.E Higgins and R.D. Ohmart. 1977.
Avian use of saltcedar communities in the
lower Colorado River valley.
In Importance,
Preservation
and Management -of
Riparian
Habitats:
a Symposium.
U.S.D.A. Forest
Service Gen. Tech. Rep. RM-43 p. 128-136.
August density
Figure 1.--Comparison of average total lizard
densities for two dates in seven habitats
along the Colorado River in Grand Canyon.
Circles indicate shoreline habitats (less
than 5 meters from shoreline), triangles
indicate riparian habitats (greater than 5
meters from shoreline), and square indicates
desertscrub.
Regression equation is y=1.28x
+ 3.75 (r=0.98, n=7).
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these
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is
important to note that these classifications refer
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Shoreline lizard densities along the Colorado
River were found to be higher than densities in
riverine riparian vegetation, which in turn were
higher than non-riparian desertscrub densities.
Shoreline densities for the four most common
species were higher than densities previously
reported for those species anywhere else in the
southwest.
The reason for the high densities
observed is probably high food availability on
riparian plants and on debris along the water's
edge.
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